The problem of saving energy by use of an exterior source of combustion air to a sealed fireplace is recognized in U.S. Pat. No. 4,213,445 to Mayo. This patent discloses the principle of supplying air from the exterior below the fireplace floor through an ash clean-out grate, which may be completely removed for adequate draft supply. An inverted metal duct having a closed rear end and leading to a forward end is disclosed. Firewood is supported on the upper horizontal duct wall, and retained by sideward slanting wing-like plates. The amount of air is controlled by sliding, into and out of the front end of the metal air duct so provided, a short duct section which is open at the top behind an air deflector plate.
The construction there illustrated is disclosed to heat the upper duct surface to a red heat, so that steel of refractory quality is required. No provision is made for accommodating ashes, nor for permitting any draft below the wood so supported directly on the tunnel surface. That disclosure suggests that the top surface of the duct plus its sideward wings may be considered as equivalent to a log basket.